Generally, an electrode assembly having a stepped portion has a structure such as that illustrated in FIG. 1. Referring to FIG. 1, an electrode assembly 1 includes positive electrodes or negative electrodes as outermost electrodes.
In the related art, an electrode assembly having a stepped portion is formed by arranging negative and positive electrodes having a first area on a long separator, folding or stacking the positive and negative electrodes together with the separator to form a first electrode stack not having a stepped portion, forming a second electrode stack having a second area in the same manner, and stacking the first and second electrode stacks.
In the electrode assembly formed as described above by a method of the related art, a slope of the separator is not formed at the stepped portion because the electrode stacks having different areas are individually formed and then stacked.
However, for reasons such as the automation of an assembling process, an electrode assembly having a stepped portion may be formed by arranging electrode units on at least one separator and folding the separator together with the electrode units. In this case, the separator is stretched to have a flat shape and brought into contact with electrodes or electrode surfaces of unit cells, and while the separator is wrapped around an already-formed electrode stack, the next electrodes or unit cells are sequentially disposed on an upper side, a lower side, or upper and lower sides of the electrode stack.
In this case, since a tensile force is applied to the separator, the separator forms a slope at the stepped portion formed between layers of the electrode assembly. However, such a slope of the separator may make it difficult to insert the electrode assembly into a battery case. In addition, during a pressing process, a lower large electrode of the electrode assembly may be bent due to tension of the separator as illustrated in FIG. 6, thereby causing shape defects of the electrode assembly. Therefore, when an electrode assembly is formed by a stacking and folding method, it is necessary to remove tension from a separator.